Eyepiece Collection Calculator

Every eyepiece's magnification is simply your telescope's focal length divided by the eyepiece's own focal length, so a shorter eyepiece always yields higher magnification and a longer one yields lower — this calculator inverts that relationship to tell you which eyepiece focal lengths to buy for three useful magnification tiers. Low power uses your minimum usable magnification (a good rule of thumb is aperture in millimeters divided by 7, which gives the widest field and brightest image for hunting down deep-sky objects), high power uses your maximum usable magnification (roughly aperture times 2, the ceiling most nights of atmospheric seeing will actually support), and medium power sits at the geometric — not arithmetic — mean of the two, which spaces the three eyepieces roughly evenly across the full useful magnification range rather than clustering them toward one end. True field of view for the low-power eyepiece estimates how much sky that eyepiece actually shows, assuming a typical eyepiece's apparent field of roughly 60 degrees; a genuine wide-field eyepiece with a larger apparent field will show noticeably more real sky at the same magnification than this estimate suggests.

This calculator only recommends focal lengths — it doesn't account for eyepiece barrel size (1.25-inch vs 2-inch), eye relief for eyeglass wearers, or a specific eyepiece design's actual apparent field of view, all of which matter when choosing real products to buy. While Scope f/ratio is a useful number for judging image brightness and field curvature, none of the math above touches it — Scope Focal Length together with your chosen magnification range is all that determines these results, independent of aperture.